Antisense technology is an effective means for reducing the expression of specific gene products and can therefore be useful in therapeutic, diagnostic, and research applications. Generally, the principle behind antisense technology is that an antisense compound (a sequence of oligonucleotides or analogues thereof) hybridizes to a target nucleic acid and modulates gene expression activities or function, such as transcription and/or translation. Regardless of the specific mechanism, its sequence-specificity makes antisense compounds attractive as tools for target validation and gene functionalization, as well as therapeutics to selectively modulate the expression of genes involved in the pathogenesis of diseases.
Chemically modified nucleosides are routinely incorporated into antisense compounds to enhance its properties, such as nuclease resistance, pharmacokinetics or affinity for a target RNA.
Chemical modifications have improved the potency and efficacy of antisense compounds, improving their potential for oral delivery or subcutaneous administration, or decreasing their potential for side effects. Chemical modifications increasing potency of antisense compounds allow administration of lower doses, which reduces the potential for toxicity. Modifications increasing the resistance to degradation result in slower clearance from the body, allowing for less frequent dosing. The synthesis of tricyclic nucleosides (Steffens et al., Helvetica Chimica Acta, 1997, 80, 2426-2439) and their incorporation into oligomeric compounds has been reported in the literature (Steffens et al., J. Am. Chem. Soc., 1997, 119, 11548-11549; Steffens et al., J. Am. Chem. Soc., 1999, 121, 3249-3255; Renneberg et al., J. Am. Chem. Soc., 2002, 124, 5993-6002; Scheidegger et al., Chem. Eur. J., 2006, 12, 8014-8023). Fully modified tricyclic oligonucleotides were shown to be more stable against nucleolytic degradation in fetal calf serum compared to unmodified oligodeoxynucleotides and to produce biological antisense effects in cellular assays, such as splice restoration of mutant β-globin (Renneberg et al., Nucleic Acids Res. 2002, 30, 2751-2757); or exon skipping in cyclophilin A (Ittig et al., Nucleic Acids Research, 2004, 32, 346-353).